Add support for including a text file in the generated email message to be used as header, footer or body

[deoren]

For example, I am using a custom Redmine field to track which node(s) are affected by a particular issue. While it is useful to have the node mentioned in the subject line, for purposes of later sorting & tracking it would be useful if this script could include a "raw" text file so users (read: sysadmins) could use custom fields that would otherwise need to be hard-coded into the script itself.

I'm unsure whether we would want to use a fixed name for the text file, pull in all files with a certain extension (in a specific directory) or read in a variable from the external conf file (for the file location).